1. Field of the Invention
This invention relates to servicing a wellbore. More specifically, it relates to servicing a wellbore with sealant compositions comprising a cationic latex and methods of using same.
2. Background of the Invention
Natural resources such as gas, oil, and water residing in a subterranean formation or zone are usually recovered by drilling a wellbore down to the subterranean formation while circulating a drilling fluid in the wellbore. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the wellbore. The drilling fluid is then usually circulated downward through the interior of the pipe and upward through the annulus, which is located between the exterior of the pipe and the walls of the wellbore. Next, primary cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus. Subsequent secondary cementing operations may also be performed.
Fluids used in servicing a wellbore may be lost to the subterranean formation while circulating the fluids in the wellbore. These fluids may enter the subterranean formation via permeable zones such as depleted zones, zones of relatively low pressure, lost circulation zones having naturally occurring fractures, weak zones having fracture gradients exceeded by the hydrostatic pressure of the servicing fluid, and so forth. As a result, the service provided by such fluids is more difficult to achieve. Also, the loss of such fluids increases the cost of the overall operation due to the prolonged rig time required, the fluids being relatively expensive, and possibly a need to install casing.
There are a variety of methodologies for combating drilling fluid circulation losses. Such methodologies may involve adding loss prevention materials to the drilling fluid itself and continue the drilling process or pump fluid until fluid circulation is restored. Similarly, cementing slurries may be lost to the formation during placement for a variety of reasons, including exceeding the fracture gradient of the formation because of high hydrostatic pressures. Specific methods for preventing fluid circulation losses include pumping cement slurries containing cement in an aqueous or nonaqueous fluid, sodium silicate solutions, or latex based fluids and causing them to mix with another appropriate fluid to form solid plugs in the loss circulation zones.
When such methods are successful in preventing drilling fluid circulation losses, the operators may case the wellbore or use a drill ahead process (vide infra). For example, operators may temporarily stop drilling operations, case the well bore, and cement the casing before resuming further drilling, which may result in a reduced well bore diameter from that point forward. Casing the wellbore is practiced when the loss circulation treatment is not strong enough to withstand hydrostatic pressure of the drilling fluid if drilling is resumed without casing the well bore. Alternatively, it is more economical during the well construction phase and more profitable during the production phase if the loss circulation treatment provides sufficient reinforcement to the loss circulation zone such that it can withstand hydrostatic pressure from further drilling without resorting to casing the wellbore. This will not only save the cost of casing, but it will also lead to a wider well bore diameter upon completion which will increase fluid production rates. The latter process is referred to as “drill ahead” process in the industry.
Another oil field related problem that occurs either during well construction or in mature oilwells is unwanted water production. While hydrocarbon-producing wells are usually completed in hydrocarbon-bearing formations, such formations may contain, or may be adjacent to, water-bearing sections. Generally, the term “water-bearing section” refers to any portion of a subterranean formation that may produce water, including a hydrocarbon-bearing section that has sufficiently high water saturation such that water may be produced along with hydrocarbons. The high mobility of the water may allow it to flow into the well bore by way of natural fractures and/or high permeability streaks present in the formation. During oilwell construction, drilling through a fresh water zone may cause water flow into the wellbore through natural or induced fractures. The production of water with hydrocarbons from subterranean wells constitutes a major problem and expense in the production of the hydrocarbons. Over the life of such wells, the ratio of water to hydrocarbons recovered may be undesirable in view of the cost of producing the water, separating it from the hydrocarbons, and disposing of it, which can represent a significant economic loss.
Subterranean stimulation treatments have long been used in the field of hydrocarbon production to increase the flow of hydrocarbons to the well bore. One such stimulation treatment is hydraulic fracturing, where specialized fluids are pumped into the subterranean formation at sufficient pressures to create or enhance at least one fracture within the formation, thereby increasing fluid flow through the formation to the well bore. When a formation contains water-bearing sections, however, the water level will increase continuously over time and may reach a point when the fracture need to be plugged and a new fracture introduced at a different depth in the wellbore. In all such cases to control the production of unwanted water through fractures, corrective fluids containing sealant compositions, for example cementitious compositions, need to be pumped into the fracture and allowed to plug them. Such methods of controlling water production are frequently referred to as conformance control. Magnesium salt based sealant systems typically referred to as Sorel cements, comprising magnesium oxide and a soluble salt, for example magnesium chloride, magnesium sulfate or ammonium mono or dibasic phosphate have been found to be suitable for such conformance control applications. Sorel cements based on magnesium chloride and magnesium sulfates as soluble salts suffer from instability upon exposure to water. The instability manifests as the development of cracks in a short time upon exposure to water and the subsequent loss of structural integrity as a sealant.
Anionic latex has been used as an additive to modify the properties of cement slurries. For cementing wells with acidic gases, such as carbon dioxide or formation fluids containing hydrogen sulfide, high alumina cements are typically employed. These cements may also be used in the cementing of high temperature wells. The use of anionic latex, for the purpose of fluid loss control, permeability reduction or mechanical property modification of high alumina cements has often resulted in cement slurries with poor fluid loss control and in set cement compositions that leach latex into the aqueous fluids surrounding the cement. Leaching of the latex by water over the lifetime of the well increases the permeability of the set cement and detrimentally affects its mechanical properties by reducing its strength and elasticity.
Accordingly, a need exists for improved wellbore sealant compositions suitable for lost circulation applications, and in particular with drill ahead operations. A need also exists for improved magnesium salt-based sealant compositions that are more resistant to structural degradation, and in particular in conformance control operations. A need also exists for improved sealant composition comprising latex that does not leach out of the sealant composition, an in particular high alumina based sealant compositions. The present disclosure addresses such needs as well an others that may be apparent to those skilled in the art.